Analog-to-digital (A/D) converters are used to convert real world continuously varying analog signals to digital data for use by digital systems that process, store, interpret, and manipulate digital data. Some applications, such as radar signal processing, communications, and instrumentation, require analog-to-digital conversion at rates exceeding that of electronic A/D converters. For these applications, electrooptic devices typically are used to perform analog-to-digital conversion.
One approach to high-speed analog-to-digital conversion uses an electrooptical modulator for optical sampling. See H. F. Taylor, "An Electrooptic Analog-to-Digital Converter, Proceedings of the IEEE, Volume 63, Pages 1524-1525, 1975. Taylor's electrooptic A/D converter uses an electrooptic modulator with an output intensity that has a periodic dependence on the voltage applied to its modulation input. The output of the modulator is captured by a photodetector and compared to a threshold signal. The result of the comparison is a digital output that represents a one bit analog-to-digital conversion of the analog voltage applied to the modulation input.
Taylor also discloses a multibit A/D converter that uses multiple modulators where the ratio of the output intensity to the input voltage of each modulator is progressively scaled by a factor of two. The speed of Taylor's electrooptic A/D converter is, however, limited by the speed of the electronic amplifiers that amplify the signals generated by the photodiodes and the speed of the electronic comparators that compare the modulator outputs to the threshold signal. Taylor's electrooptic A/D converter also requires the use of electrooptic modulators that have precise predetermined gain.
U.S. Pat. No. 5,010,346 describes another approach to high-speed analog-to-digital conversion that uses a time-demultiplexing architecture. The device uses electrooptic modulators to generate modulated light beams from an input pulsed light beam and from an input analog signal. An optical demultiplexer is used to demultiplex the modulated light beam to multiple paths to reduce the pulse repetition rate. A photodetector is coupled to each of the outputs of the demultiplexer to convert the demultiplexed modulated light beams into packets of charge. Electronic A/D converters are used to quantize the charge packets thereby producing a digital signal.
The A/D converter described in U.S. Pat. No. 5,010,346 requires the use of a large number of electronic A/D converters to achieve high-speed analog-to-digital conversion. Using a large number of electronic A/D converters may not be desirable because electronic A/D converters are relatively large and complex devices that are relatively expensive and difficult to manufacture. In addition, the A/D converter described in U.S. Pat. No. 5,010,346 requires the use of a pulsed laser.
There exists a need for a high-speed A/D converter that is not limited by the speed of the electronic devices such as amplifiers and comparators and that does not require the use of a large number of electronic A/D converters.